Wastewater treatment plants often use lamp racks oriented horizontally in the direction of flow in an open channel. The lamps emit ultraviolet light (UV) that inactivates pathogenic microorganisms rendering the water safe for discharge to a receiving water body or for re-use of the water (irrigation, indirect potable re-use, industrial use, gray water for non-potable use, etc.) The racks hold lamps in an array dispersed over the cross section of the channel such that none of the water flowing down the channel passes too far from any one lamp. Known open channel fluid treatment devices are shown, by example, in U.S. Pat. Nos. 4,482,809; and 5,006,244 the disclosures of which are incorporated by reference herein.
There is a practical limit on how far water can pass from a lamp and still receive adequate disinfection. FIG. 1 is a chart showing the drop off in UV irradiance with distance from the lamp in water with UV transmittance of 55% T and 65% T.
Typically UV systems using low pressure mercury arc lamps have a lamp spacing of approximately 7.5 cm in a square array. With 2.5 cm diameter quartz tubes this means that the maximum distance from any lamp is approximately 4 cm. This provides sufficient space for the water to pass between the lamps without undue head loss and is close enough to achieve adequate penetration of the UV to all areas and hence adequate disinfection. These low pressure systems have lamps with a total power consumption of under 100 Watts and a UVC (germicidal UV) output of under 50 Watts.
More recent advancement in lamp technology has produced low pressure lamps with higher output. Higher lamp output means that more water can be disinfected per lamp, and hence the flow of water must be increased proportional to the lamp UVC output. However due to head loss limits across a bank of lamps (too high a head loss means that the level of water upstream of the bank must increase and some of the water will spill over the top of the lamp bank and not be adequately treated), the lamp spacing must be increased to accommodate the greater water flow. For example lamps with an electrical consumption of 250 Watts and UVC output of approximately 100 Watts, must be accommodated in arrays with 10 cm lamp spacing. The additional area for the flow of water limits the velocity and hence head loss across the lamp bank. This results in a reduction in the UV irradiance at the point furthest from all the lamps as shown in FIG. 2.
This reduced irradiance at the furthest point from the lamps results in some decrease in the performance efficiency associated with this greater lamp spacing, especially at lower UV Transmittances (55% T), but the advantages of being able to use fewer lamps overcomes the increase in electrical consumption that results.
More recent development of even higher powered lamps (500 Watt, with 200 W UVC output) would potentially result in the number of lamps needed being reduced to half that of systems employing 250 W lamps. However this means that the flow per lamp must be doubled, resulting in a quadrupling in the head loss across a lamp bank (head loss is proportional velocity squared) unless the spacing of the lamps is increased even more. However increasing the spacing beyond 10 cm results in a further reduction in treatment efficiency, negating the potential advantages of fewer higher power lamps.
One means of overcoming this is to close off the top of the lamp bank such that water cannot spill over the top of the bank and is forced to flow at the higher velocity and consequent pressure loss past the lamps with the smaller 4 inch or lower lamp spacing. This has been successfully employed where much higher powered medium pressure (MP) lamps are used (2500 Watt/lamp, 370′Wall UVC) (U.S. Pat. No. 5,590,390, the disclosure of which is incorporated by reference herein) and in a system that employed triangularly shaped or delta wing mixing elements with even greater spacing and 5,000 Watt lamps (750 Watt UVC) (U.S. Pat. No. 6,015,229, the disclosure of which is incorporated by reference herein). Even though the system disclosed in U.S. Pat. No. 6,015,229 had the closed top, the lamp spacing still had to be increased to reduce overall velocity and head loss. In the system disclosed in U.S. Pat. No. 6,015,229, the 5,000 Watt MP lamps are relatively short (60 cm long). One drawback of the system disclosed in U.S. Pat. No. 6,015,229 is that if longer lamps are used, the vortices generated by the delta wings die off and the effectiveness is diminished. The system disclosed in U.S. Pat. No. 6,015,229 therefore is best used with relatively short MP lamps (60 cm long vs. typical 1.8 m long for LP lamps).
With one delta wing array placed at the beginning of a LP lamp bank the vortices essentially die out after approximately 40 cm. This has been modeled using Computational Fluid Dynamic Modeling (CFD) and is shown in FIGS. 3 and 4. FIG. 3 is a velocity vortex diagram showing vortices 2 cm downstream of delta wings. FIG. 4 is a velocity vortex diagram showing vortices 40 cm downstream of delta wings.
The rotational velocity and therefore ability of the vortices to mix in the water furthest from the lamps is represented by the velocity vectors in FIGS. 3 and 4, with longer arrows and therefore higher rotational speeds immediately after the lamp (FIG. 3) and smaller arrows and hence lower rotational speed 40 cm downstream of the deltas.